This invention relates generally to methods and apparatus for transfer and detac polarity switching for rendering latent electrostatic images visible in systems having optional colors of dry toner or developer and, more particularly, to printing toner images in a single pass of the imaging surface through the processing areas of the printing apparatus upon operator selection among multiple highlighting colors by automatically adjusting transfer and detac corotron polarities in response to the operator selection.
Modern business and computer needs oftentimes make it advantageous and desirable to reproduce multi color originals into copies that contain colors selected from a plurality of highlight colors.
Several useful methods are known for making copies having highlight colors. Various techniques for controlling and switching transfer and detac corotrons are also well known. For example,
U.S. Pat. No. 4,190,962 to Friday, assigned to Xerox Corporation, describes an apparatus and process by which higher transfer efficiencies and better image transfer can be achieved for the lead edge of a copy sheet. Increased transfer charge is applied to the lead edge area of the copy sheet to provide increased transfer electrostatic field to that area in proportion to the remainder of the copy sheet, prior to the copy sheet being effectively neutralized for stripping in the lead edge area by a detacking corona generator. See FIG. 1 and Col. 4, lines 30-60 for a detack corotron power supply and switching the output of the detack corotron.
U.S. Pat. No. 4,791,528 to Suzuki et al. relates to a power supply device capable of supplying high and low voltages for photocopying machines. A switched input voltage is applied to a self excited transformer having two secondary output voltages.
U.S. Pat. No. 4,714,978 to Coleman, assigned to Xerox Corporation, describes a circuit for supplying constant power to A.C. Corotrons. The circuit consists of a pulse width modulator which provides alternate drive signals of adjustable pulse width at a constant frequency; a transformer; a switch pair for separably connecting a common d.c. power source to a transformer primary winding in response to drive signals from the modulator to induce an a.c. output in a transformer secondary winding to a corotron. The secondary winding of the transformer is connected by line to leads of detack and pre-charge corotrons. To provide constant current output to detack and pre-charge corotrons, the operating current of corotrons is monitored using a feedback loop which employs a voltage doubler network consisting of capacitors and diodes for sensing corotron current.
U.S. Pat. No. 4,140,962 to Quinn, assigned to Xerox Corporation, describes an electrical power regulator for a corona discharge device in which a high voltage electrical voltage supply is connected to a corona discharge device having a wire extending in spaced relation relative both to a plate and to a shield which is electrically connected in series with unidirectional current blocking means arranged in mutually opposed polarity to conduct current in opposite directions between the shield and the plate.
Japanese Patent No. 58-184169 to Yamada relates to an electrophotographic device, in which selective switching of modes is facilitated by applying an electric charge to a photoreceptor and charging the photoreceptor with the opposite polarity by a voltage lower than the electric charge to expose an image.
Japanese Patent No. 61-201281 to Tanaka et al. describes a method to prevent specified ions from remaining nearby a photosensitive body by connecting a high plus voltage power source which applies a high plus voltage and a high minus voltage power source which applies a high minus voltage to the discharge wire of a corotron for transfer in parallel, and which applies a voltage of the opposite polarity from toner during transfer operation and a voltage of the same polarity with the toner when transfer operation is not performed.
However, a difficulty with the prior art systems is either the limitation with respect to colors or the need for multiple copy sheet passes through the system and multiple development passes of the photoreceptor to achieve color on the copy sheet. In particular, it is desirable to be able to print images having multiple colors or being limited by the need for multiple passes through the system for successive transfer of different color toners. It would be desirable, therefore, to be able to produce highlight color copies in a single pass of the photoreceptor or other charge retentive surface past the printing process stations.
It is an object, therefore, of the present invention to provide a new and improved system for providing multiple colors in a single pass of the copy sheet through a transfer station. Another object of the present invention is to provide a reliable, less complicated switching mechanism for automatically adjusting the polarities and potentials of transfer and detac corotrons in response to optionally selected colors in a single pass machine. Another object of the present invention is to provide multiple colors in a single pass color machine by automatically altering a corona generating device polarity in response to selected colors. Another object of the present invention is to automatically respond to operator selection of a particular color by switching the polarity of a transfer corotron and automatically adjusting the output of the detac corona device. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.